The present invention relates to a circuit for the transmission of communication signals between a central office and two subscribers which are connected to the central office via a common subscriber line, in which the signals are transmitted from the central office in digital form over the subscriber line to a device in which the incoming digital signals are converted into analog signals and vice versa, the two subscribers being connected to the conversion device over cables.
Such a circuit arrangement is used, for instance, in the "PCM-2" method employed by the Post Office of the Federal Republic of Germany. In that method, two spatially adjacent subscribers to the public telecommunication network are both connected to the office by the same subscriber line. The letters "PCM" stand for the well known pulse code modulation method, by which communication signals are transmitted as digital signals.
In the PCM-2 method, the analog signals are digitalized in the office and sent out on the subscriber line. At the end of the subscriber line there is installed a device by which the incoming digital signals are converted back into analog signals. This is true, of course, also of the opposite direction. The two subscribers are connected to the device by cables. Because of the digital transmission of the signals, the two subscribers can telephone simultaneously without interfering with each other. The two flows of signals are "interlaced" by the well-known technique at the starting end of the subscriber line and then separated again at the end thereof. The two subscribers therefore need not know that they are both using the same subscriber line.
No problems arise with this system as long as the device operates free of disturbance. Disturbances of the device are also unproblematical if the device is accessible at all times. However, difficulties result if disturbances cannot be repaired rapidly and if access cannot be readily had to the device. This is the case when the device is installed on the premises of one of the subscribers, as will normally be the case because of considerations of cost and security. It may then happen that the device is not accessible for a lengthy period of time, for instance during the absence of that subscriber on vacation. If trouble in the device occurs during such time, this disturbance cannot be eliminated. The one subscriber, therefore, cannot telephone as long as the subscriber at whose premises the device is installed is still on vacation. In addition to this, even if the subscriber at whose premises the device is installed is there, it may be difficult to get at the device if only the telephone of the other subscriber is experiencing trouble. The telephone of the subscriber who has the device may not be affected thereby. He then does not see any necessity, without lengthy explanations, of permitting a repairman to enter his premises.